Experimental constraints on the rheology, eruption and emplacement dynamics of analog lavas comparable to Mercurys northern volcanic plains

Corresponding author: Francesco Vetere (francesco.vetere@unipg.it)

Key Points: • New viscosity data for Mercury northern volcanic plains lavas are presented.
• Mercury lavas show shear thinning behaviour with a decrease of viscosity of ca. 1 log unit as shear rate (!) varies from 0.1 to 5.0 s-1. • Heat loss during lava flow and emplacement implies that high effusion rates, >10000 m3/s, are required to cover large distances as observed by MESSENGER (NASA).

Abstract We present new viscosity measurements of a synthetic silicate system considered an analogue for the lava erupted on the surface of Mercury. In particular, we focus on the northern volcanic plains (NVP), which correspond to the largest lava flows on Mercury and possibly in the Solar System. High-temperature viscosity measurements were performed at both superliquidus (up to 1736 K) and subliquidus conditions (1569–1502 K) to constrain the viscosity variations as a function of crystallinity (from 0 to 28%) and shear rate (from 0.1 to 5 s-1). Melt viscosity shows moderate variations (4 –16 Pa s) in the temperature range 1736–1600 K. Experiments performed below the liquidus temperature show a decreases in viscosity as shear rate increases from 0.1 to 5 s-1, resulting in a shear thinning behaviour, with a decrease in viscosity of ca. 1 log unit. The low viscosity of the studied composition may explain the ability of NVP lavas to cover long distances, on the order of hundreds of kilometres in a turbulent flow regime. Using our experimental data we estimate that lava flows with thickness of 1, 5 and 10 m are likely to have velocities of 4.8, 6.5 and 7.2 m/s respectively, on a 5° ground slope. Numerical modelling incorporating both the heat loss of the lavas and its possible crystallization during emplacement allows us to infer that high effusion rates (> 10000 m3/s) are necessary to cover the large distances indicated by satellite data from the MESSENGER spacecraft.

1 Introduction The eccentricity of the orbit of Mercury, in combination with the planet’s vicinity to the Sun, is responsible for its very long days (~ 59 terrestrial daytimes) and, locally, extremely high surface temperatures. The daylight temperature at perihelion, estimated on the surface at the equator, is ~700 K, whereas it decreases to ~350 K at 85°N. During the night, the lack of a shielding atmosphere produces a high loss of thermal energy due to radiation and temperature decreases to ~100 K [Paige et al., 1992; Vasavada et al., 1999]. The surface of Mercury is dominated by a secondary volcanic crust, the majority of which formed between 4.2 and 3.5 Ga [Head et al., 2011; Weider et al., 2012; Denevi et al., 2013; Byrne et al., 2016], with minor explosive volcanic activity until ~ 1.0 Ga [Thomas et al., 2014]. Geochemical mapping using the X-Ray Spectrometer (XRS) and Gamma-Ray Spectrometer (GRS) of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft [Solomon et al., 2001] revealed that the volcanic crust is Mg-rich and Al- and Ca-poor in comparison with terrestrial and lunar crustal material [Nittler et al., 2011; Weider et al., 2012, 2015; Peplowski et al., 2015]. Mercury’s crust is also strongly depleted in Fe [Izenberg et al., 2014; Weider et al., 2015]. This is most likely due to extreme partitioning of iron into the core [Hauck et al., 2013] during early differentiation of the planets under highly reducing conditions (IW-3 to IW-7 with IW being the iron- wüstite oxygen fugacity buffer)
[Malavergne et al., 2010; McCubbin et al., 2012; Zolotov et al., 2013; Namur et al., 2016a]. The extremely high sulfur contents measured by MESSENGER (1–3 wt.%; [Weider et al., 2015]) can also be explained by differentiation under reducing conditions [Namur et al., 2016a], as sulfur solubility in silicate melts increases with progressively reduced oxygen fugacity conditions

[McCoy et al., 1999; Berthet et al., 2009; Zolotov et al., 2013; Cartier et al., 2014; Namur et al., 2016a]. The largest effusive events on Mercury occurred at the highest latitudes of the northern hemisphere and are represented by lavas with the highest SiO2- and Al2O3-contents and the lowest MgO-contents detected on the planet [Weider et al., 2015; Namur et al., 2016b]. These lavas belong to a single smooth plain deposit referred to as the northern volcanic

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